Machine vision technology is now being employed in many industries for the purpose of rapidly accomplishing inspection of articles with very high accuracy. Within the electronics industry, machine vision systems are currently being employed to inspect populated circuit boards prior to soldering to detect missing and misplaced components as well as to inspect unpopulated boards prior to component placement to detect missing and misplaced solder paste deposits. In U.S. Pat. No. 4,811,410, issued on Mar. 7, 1989, in the names of I. Amir et al. (herein incorporated by reference), there is disclosed a circuit board inspection system comprised of a light source for illuminating a strip of area running across the surface of the board with light of substantially constant intensity. A linescan camera is provided for detecting the intensity of the light reflected from the illuminated strip. The output signal of the camera is processed by a vision system which initially serves to winnow the data so that only the data representing the image of predetermined regions of interest in the strip (where defects are more likely) is retained. The retained data is then processed to detect the presence of defects, if any, in the strip. By simultaneously spanning the circuit board with the light source and the camera, the entire surface of the board can thus be inspected for defects.
While the Amir et al. vision system has proven itself capable of detecting defects such as missing and misplaced components and solder paste deposits, a lack of contrast between the features on the board (i.e., the components and/or solder paste deposits) can adversely affect the accuracy of inspection which is also the case for other conventional inspection systems. Often, the features of interest on the circuit board have a very low contrast with respect to the background and, as a consequence, the intensity of the light reflected from such features is often not appreciably greater than the light reflected from the board itself. Because the contrast between the circuit board and the features of interest is usually not very high, defect detection is difficult.
In an effort to overcome this problem, three-dimensional imaging techniques have been proposed. My co-pending application, Ser. No. 440,948, filed on Nov. 24, 1989, discloses a three-dimensional imaging system, which in a preferred embodiment, comprises a pair of light sources which are separately rendered operative to illuminate a separate one of each first and second spaced-apart strips of area, respectively, on a substrate (e.g., a circuit board) with light. A linescan camera is positioned above the substrate to capture the image of a third strip of area, lying between the first and second strips. When each of the first and second strips is illuminated by a separate one of the first and second lines of light, respectively, the third strip is illuminated with light having a gradient profile. The height of the features (if any) in the third strip can be obtained from the ratio of the sensed reflectance intensities attributed to the first and second light sources, respectively. By separately spanning the substrate with each of the first and second light sources, a three-dimensional image of the entire substrate can be obtained.
While the three-dimensional imaging technique disclosed in my co-pending '948 application advantageously yields information about the height of the features on a circuit board, thereby enabling the presence and position of such features to be more easily established, obtaining a three-dimensional image generally requires additional hardware. Moreover, the three-dimensional imaging systems disclosed in my '948 application may take longer under certain conditions, to produce a three-dimensional image, as compared to the time required for the vision system taught in the '410 system to produce a two-dimensional image.
Thus, there is a need for an improved two-dimensional imaging technique which advantageously enhances the boundary region between an object and its surrounding background to facilitate detection of the presence and position of the object.